Use of laser interferometry is known for precise measurement of small displacements and rotations using a beam of light split into reference and measurement beams. The measurement beam is reflected from a mirror mounted on a movable object and is combined with the reference beam reflected from a stationary object to generate a phase difference. The phase difference is proportional to the amount of displacement made between the reflector of the reference beam and the reflector of the measurement beam.
In many applications, the measurement beam path passes through air. In some applications, a double pass beam path can permit cancellation of propagation angles between the measurement beam path and the reference beam path. The double pass beam path increases the beam path length. Furthermore, the optical path length of the light can be more than four times the stroke of the moving object. In wafer exposure tools used in integrated circuit lithography, the optical path length can approach two meters. The long beam path presents a challenge for the measurement because the air through which the light travels must be well controlled in terms of temperature, pressure and gas composition. Because temperature, pressure and gas composition all affect a refractive index of air, a change or inconsistency in any of them presents as a phase shift and therefore a measurement error or measurement uncertainty. In many applications, the control of the index of refraction of air is becoming a performance limiting factor.
Among other considerations, it is desirable to reduce a path length of light travel in interferometry displacement measurement applications.